If you install Debian on a computer with an AMD graphic card (an integrated Radeon HD 6310 in my case), you will only get garbage on your screen. Even without X.Org.
This is because these graphic card require a non-free firmware to work, even in 2D with free drivers. So, boot with the kernel option
radeon.modeset=0 to get a working console, then install the non-free package linux-firmware-nonfree.
After that, do not forget to complain to AMD, because that really sucks hard. At the beginning there were microwave network cards unusable without non-free firmware, then wired network cards, and they are even invading graphic cards!